_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
of methylcopper formed as an intermediate. Alternatively chemisorbed CH,Cl
may react with activated silicon or silicide phases such as Cu,Si. A typical product
mixture consists of (%. b.p.) Me,SiCl 2 (75. 70°C}, MeSiCl 3 (10, 66°C},
Me 3 SiCI (4, 58°C), MeSiHCI 2 (6, 41 °C} and smaller quantities of SiCI 4 , Me 4 Si,
HSiCI 3 and disilanes. On account of the close boiling points of these materials
careful fractionation is necessary to separate them.
The direct synthesis also works with methyl bromide, but methyl fluoride and
methyl iodide do not react satisfactorily. Phenylchlorosilanes, however, can be
prepared starting from chlorobenzene. Other related reactions include:
3MeOH +Si ----> (MeO),SiH + H, (catalysed by copper. no migration of catalyst occurs)
3HCI +Si----> HSiCI 3 + H, (no catalyst required).
Redistribution of groups between silicon occurs approximately statistically,
because the L\Go values for the reactions are very small (ca. + 4 to - 4 kJ moi- 1 ).
SiX 4 + SiY 4 ~ SiX4-n Y n
Exchange between 'labile' groups (X, Y = halogen, OR, SR, NR,) occurs rapidly in
the presence of acid or base catalysts, so that the most volatile component can be
distilled out preferentially. For redistributions involving Si-C bonds, strong Lewis
acid catalysts such as AICI 3 or Al,0 3 are required. These reactions involve
electrophilic substitution at carbon and do not proceed through organoaluminium
intermediates. Methylchlorosilanes obtained by the direct process are
interconverted commercially in this way e.g.
AICI,
MeSiCI, + Me,SiCI ~ Me,SiCI,
4.4.2 Hydrolysis of chlorosilanes-silanols
Hydrolysis of alkyl or arylchlorosilanes initially yields silanols R 3 Si(OH),
R,Si(OH), or RSi(OH) 3 • Except where R is a bulky group such as tert-buty! or
cyclohexyl, these compounds are very prone to self-condensation, giving
siloxanes. Consideration is brought about by traces of acids or bases. so it is
essential to ensure carefully buffered neutra! conditions in their preparation.
Me,SiCI,
H 2 0,AI 2 (S0 4 ) 3
MgCO,
Me,Si(OH),----> HOSiMe,(OSiMe 2 )"0SiMe 2 0H
m.p. l00°C
Silanols are associated through intermolecular hydrogen bonding. Solid diethylsilane
diol, m.p. 96° has a structure in which hydrogen bonded chains are
further linked to neighbouring chains, also through hydrogen bonds, to form
layers.
Me 3 SiOH and Ph 3 SiOH are more strongly acidic than the corresponding
alcohols Me 3 COH and Ph 3 COH. Ph 3 SiOH is about as strong an acid as phenol.
This is surprising, as silicon is less electronegative than carbon; inductive effects
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